The light emitting diode (LED) lighting market is expanding as the cost of LED lighting decreases. As compared to incandescent lamps, fluorescent lamps, and other light sources, LEDs have a longer service life and can be dimmed without impairing the service life of the LED. Dimmable LED drivers with wide range dimming capability are desirable.
There are two basic methods to accomplish the dimming function in LED driver circuits. In buck converter and buck-boost converter LED driver circuits, pulse width modulation duty cycle variation is used to reduce the output current to the LED (i.e., dim the LED light source). In half-bridge resonant type driver circuits (which are often isolated), frequency control of the half-bridge resonant inverter is used to reduce output current to the LED light source. Both driver circuit topologies and methods have hard-switching problems and frequency limiting problems when dimming. This makes switch (e.g., MOSFET) selection difficult (e.g., balancing the competing interests of cost, size, and robustness) and the switches selected may reduce circuit efficiency and/or reliability.
Referring to FIG. 1, a prior art frequency controlled half-bridge resonant type driver circuit 100 works well for certain range dimming control. By changing the switching frequency of the half-bridge inverter 102, the resonant tank circuit 104 output can be predictably altered and controlled. As the dimming range is increased, the switching frequency range of the half-bridge must also be increased. However, if the switching frequency of the half-bridge inverter 102 is too high, there is not enough current going through the resonant inductor Lres to maintain soft-switching. To maintain soft-switching in the half-bridge inverter 102, the switching frequency must be limited. Thus, there is a practical maximum frequency limit and corresponding minimum dimming level for driver circuits based on the half-bridge resonant topology.